During subsea operations, it is frequently necessary to accurately identify the position and or orientation of an object, either in absolute terms or relative to another object. Such objects may include oil pipelines and flanges, engineering structures, geological structures, salvage items and the like. Presently, much of the work in this context takes place using remotely operated vehicles (ROVs). Nevertheless, it will be understood that similar issues apply in the case of human divers, manned submarine devices and unmanned Autonomous Underwater Vehicles (AUV's) and the invention is equally applicable to all underwater rovers.
In general, for a rover, precise absolute positioning underwater in a global reference frame is difficult, since satellite positioning systems are ineffective underwater. Unaided Inertial Navigation systems (INS) using gyroscopes and accelerometers can provide reliable coarse positioning but are subject to drift. For metrology purposes, such systems do not reliably achieve the required centimeter level relative accuracy over the period of time required to travel from a first object to a second object, without additional measures being taken. Acoustic positioning systems are effective underwater, but have disadvantages depending on the type of system and the water depth:                For USBL (Ultra Short Baseline) systems fitted on the surface vessel, the positioning accuracy of the rover degrades with increasing water depth;        LBL (Long Baseline) arrays of transponders on the seabed are expensive and further require costly deployment and calibration of the array;        Acoustic DVL (Doppler Velocity Log) aided INS positioning increases INS accuracy but nevertheless will exhibit drift;        Acoustic noise from the ROV or other sensors and multipath effects can affect the accuracy and reliability of acoustic positioning systems.        
Relative positioning is also difficult e.g. where it is desired to determine the position of a first object with respect to a second object. If the optical visibility is such that both objects are visible from a given location of the rover, it may be possible to perform optical or laser range-finding of the respective objects. Propagation of light underwater has, however, serious limitations compared to propagation in air or free space and only green to blue light can propagate a substantial distance (10's to 100's of meters) without being attenuated beyond practical use. In most situations, both objects will not be adequately visible from the same position and displacing the rover from a first position to a second position requires knowledge of the relevant displacement. Since both observations are not simultaneous, the relevant time of observation must also be taken into account
One method of underwater metrology uses cameras mounted on an ROV to take sequences of photographs from different locations. By combining the photographs using principles of triangulation, the relative positions of the surveyed objects can be determined. This procedure is known from photogrammetry and requires relatively good visibility and significant processing power. Light sources on the ROV are required when the scene is dark. This in turn further reduces visibility due to backscattering. Markings may be provided on the surveyed objects in order to improve accuracy and scaling bars are used to provide baseline measurements. Document GB2257250 describes the use of photogrammetry for underwater surveying. A further device is described in WO2011143622, which captures panoramic images underwater.
It would be desirable to provide an alternative system and method that provides additional position data, in particular as an additional redundancy to existing systems. It would be further desirable to provide a system capable of operating even in reduced visibility conditions and that efficiently uses processor capacity.